Install a password font on your computer and configure FileMaker to use it (if on Windows).

Or

Use show custom dialog and configure one of its input fields to be used with the password option.

or

use OnObjectKeystroke to capture each keystroke and store it in a different field or a variable. Have the script echo a bullet character to the visible field for each keystroke. This will require adding code to recognize and process typical editing keystrokes such as the forward and back delete buttons.